An Egyptian man reads a newspaper at a coffee shop in Cairo, Egypt, Sunday, Dec. 23, 2012. Egypt's opposition called Sunday for an investigation into allegations of vote fraud in the referendum on a deeply divisive Islamist-backed constitution after the Muslim Brotherhood, the main group backing the charter, claimed it passed with a 64 percent "yes" vote. Official results have not been released yet and are expected on Monday. (AP Photo/Amr Nabil)

Photo: Amr Nabil, Associated Press

An Egyptian man reads a newspaper at a coffee shop in Cairo, Egypt,...

With the passage of a divisive constitution, Egypt's Islamist leadership has secured its tightest grip on power since Hosni Mubarak's ouster nearly two years ago and laid the foundation for legislation to create a more religious state.

The opposition's response - a vow to keep fighting the charter and the program of President Mohammed Morsi - ensures that the turmoil of the past two years will not end as many, especially the tens of millions of poor craving stability, had fervently hoped.

"The referendum is not the end game. It is only a battle in this long struggle for the future of Egypt," the opposition National Salvation Front said in a statement Sunday. "We will not allow a change to the identity of Egypt or the return of the age of tyranny."

Critics say the new constitution does not sufficiently protect the rights of women and minority groups and empowers Muslim clerics by giving them a say over legislation. Some articles were also seen as tailored to get rid of Islamists' enemies and undermine the freedom of labor unions.

Morsi's Muslim Brotherhood, Egypt's most powerful political organization in the post-Mubarak era, claimed early Sunday that the charter it had backed was approved in the two-stage vote with a 64 percent "yes" vote overall. Official results are expected Monday.

Once the result is official, Morsi is expected to call for a new election of parliament's lawmaking lower house within two months. And if all of the elections since Mubarak's February 2011 ouster are any predictor, Islamists will again emerge dominant. In the last parliamentary vote in late 2011, the Muslim Brotherhood and its allies the Salafis - ultraconservative Islamists - won about 70 percent of seats.

Egypt analyst Michael Hanna said, however, that enduring political tensions will make it difficult for the Islamists to push ahead with any major or sensitive legislation, such as laws restricting outspoken media organizations.

"There will be a huge domestic backlash to any unpopular legislation, especially when it comes to the economy or the media," said Hanna of New York's Century Foundation.

The president will soon have to introduce painful economic reforms to salvage a deal with the International Monetary Fund for a $4.8 billion loan that was delayed at Egypt's request because of the political turmoil of the past month.